U.S. patent number 4,803,637 [Application Number 07/070,188] was granted by the patent office on 1989-02-07 for cruise control system for a vehicle.
This patent grant is currently assigned to Toyota Jidosha Kabushiki Kaisha. Invention is credited to Masumi Nagasaka, Hiroaki Tabuchi, Tetsuya Tada, Tatsuo Teratani.
United States Patent |
4,803,637 |
Tada , et al. |
February 7, 1989 |
Cruise control system for a vehicle
Abstract
A cruise control system for a vehicle having an internal
combustion engine is provided with a throttle valve operated by a
diaphragm actuator irrespective of a depression of an accelerator
pedal by an operator of the vehicle. The system is provided with a
control circuit for controlling an electric signal applied to the
actuator. A fixed level signal is first applied to the actuator for
a short period after a cruise mode operation has commenced, so that
the diaphragm is quickly moved to prevent a temporary decrease in
the vehicle speed. After the elapse of the predetermined time, the
level of the electric signal is controlled by feedback so that the
vehicle speed is controlled to a target speed. A feedback gain
initially has a first value for a period after the feedback control
is commenced and then has a second value which is smaller than the
first value. Furthermore, the feedback gain may have a third value
at a transient state from a condition where the feedback gain has
the first value and a condition where the feedback gain has the
second value.
Inventors: |
Tada; Tetsuya (Toyota,
JP), Nagasaka; Masumi (Toyota, JP),
Tabuchi; Hiroaki (Toyota, JP), Teratani; Tatsuo
(Aichi, JP) |
Assignee: |
Toyota Jidosha Kabushiki Kaisha
(Aichi, JP)
|
Family
ID: |
26356492 |
Appl.
No.: |
07/070,188 |
Filed: |
July 6, 1987 |
Foreign Application Priority Data
|
|
|
|
|
Jul 17, 1986 [JP] |
|
|
61-166803 |
Jan 31, 1987 [JP] |
|
|
62-19641 |
|
Current U.S.
Class: |
701/93; 123/352;
701/110; 180/179 |
Current CPC
Class: |
B60K
31/107 (20130101); B60W 2050/0022 (20130101) |
Current International
Class: |
B60K
31/10 (20060101); B60K 31/06 (20060101); B60K
031/00 () |
Field of
Search: |
;364/426,431.07,431.04,161,162,163,165 ;123/351,352
;180/176,177,178,179 ;318/594 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Chin; Gary
Attorney, Agent or Firm: Parkhurst, Oliff & Berridge
Claims
We claim:
1. A cruise control system for a vehicle having an internal
combustion engine, an engine output control member, and an
accelerator pedal connected to said engine output control member
for operating said engine output control member in accordance with
a requirement by an operator, said system comprising:
actuator means connected to said engine output control member for
controlling engine output without operation of said accelerator
pedal by an operator;
first detecting means for detecting an actual vehicle speed;
target value setting means for setting a target vehicle speed to be
maintained by said cruise control system;
second detecting means for detecting commencement of a cruise
control mode of operation;
electric signal forming means for forming electric signals for
operating the actuator means and thereby controlling engine output
and vehicle speed;
timer means for detecting first and second predetermined time
lapses after detection of commencement of the cruise control mode
of operation by said second detecting means;
first control means for controlling said electric signal forming
means, prior to detection of said first predetermined time lapse by
said timer means, so as to form a fixed level electric signal for
quickly moving the actuator means to prevent a temporary decrease
of the vehicle speed;
second control means for calculating feedback factors based upon
deviations between the actual vehicle speed detected by said first
detecting means and the target vehicle speed set by said target
value setting means, and for controlling said electric signal
forming means, after detection of said first predetermined time
lapse by said timer means, so as to form electric signals which are
modified by said feedback factors; and
feedback gain control means for controlling gain of said feedback
factors so that a value of said gain deceases after detection of
said second predetermined time lapse by said timer means, whereby a
quick and stable feedback control is realized.
2. A system according to claim 1, wherein said feedback gain
control means comprises:
first feedback gain control means for controlling said gain of said
feedback factors so that said gain has a first predetermined value
before detection of said second predetermined time lapse by said
timer means, in order to attain a quick control of the vehicle
speed to the target speed; and
second feedback gain control means for controlling said gain of
said feedback factors so that said gain has a second predetermined
value smaller than the first predetermined value after detection of
said predetermined time lapse by said timer means, in order to
attain stable feedback control when the actual vehicle speed is
close to the target vehicle speed.
3. A system according to claim 2, wherein upon detection of said
second predetermined time lapse by said timer means, said gain is
immediately changed from said first predetermined value to said
second predetermined value.
4. A cruise control system according to claim 2, further
comprising:
third feedback gain control means for controlling said gain of said
feedback factors so that said gain has a third predetermined value
between the first and second predetermined values;
wherein said gain has said third predetermined value after said
second predetermined time lapse and before a third predetermined
time lapse detected by said timer means, and said gain has said
second predetermined value after said third predetermined time
lapse, in order to prevent a temporary drop in vehicle speed after
detection of the second predetermined time lapse.
5. A cruise control system according to claim 1, wherein said
actuator means comprises:
a vacuum actuator connected to said engine output control member;
and
vacuum control means responsive to said electric signals for
controlling a vacuum level in the vacuum actuator for controlling
vehicle speed.
6. A cruise control system according to claim 1, wherein said
electric signal forming means produces pulse signals, and a duty
ratio of the pulse signals corresponds to said gain of said
feedback factors.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cruise control system for a
vehicle capable of controlling any deviation to a value as small as
possible.
2. Description of the Related Art
Known in the prior art is a cruise control system for an automobile
provided with an internal combustion engine, wherein the cruise
control system has an actuator for operating an output control
member of an engine, such as a throttle valve, in such a manner
that the vehicle speed is controlled to a target value when the
system is in a cruise mode. The cruise control system includes a
sensor, for detecting an actual speed of the vehicle, and means are
provided for detecting a deviation of an actual vehicle speed from
a target speed. The actuator is, for example, a vacuum actuator
which is controlled by feedback to reduce the deviation. This type
of system suffers from a drawback in that the engine speed
temporarily drops when the cruise mode operation is commenced
because of a delay in the response of the actuator. When the
control of the actuator is moved from the accelerator pedal
operated by the operator to the actuator, the throttle valve is
temporarily closed, since the actuator operates slowly. Due to this
closure of the throttle valve, a temporary decrease in vehicle
speed occurs.
In order to prevent this decrease in vehicle speed when the cruise
mode operation is commenced, a system has been proposed for quick
control of the stroke of the actuator to a position close to a
target position for obtaining the target vehicle speed. In this
system, a fixed level signal is sent to the actuator for a short
period after the cruise mode has commenced, so that the actuator is
moved to the target position at the maximum speed (see Japanese
Unexamined Patent Publication (Kokai) No. 61-44033). In this prior
patent, the actuator is quickly operated so that the temporary
closure movement of the throttle valve is made as small as
possible. Then the feedback control is commenced to control the
vehicle speed to the target value. However, this improved system
still has a drawback in that there is a certain time delay before
realizing the target stroke of the actuator which will allow the
opening of the throttle valve after the feedback control is
commenced. As a result, the vehicle speed is still temporarily
decreased after the cruise mode operation has commenced.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a cruise control
system for a vehicle, which can prevent a temporary decrease of the
vehicle speed upon commencement of the cruise mode operation.
According to the present invention, a cruise control system is
provided for a vehicle having an internal combustion engine having
means, such as a throttle valve, for controlling the output of the
engine and an accelerator pedal connected to that engine output
control means for operating same in accordance with a requirement
of an operator, the system comprising:
actuator means connected to said means for controlling the engine
output irrespective of the operation of the accelerator pedal by
the operator;
first detecting means for detection of an actual vehicle speed;
target value setting means for setting a target vehicle speed;
second detecting means for detecting a commencement of the cruise
mode operation;
electric signal forming means for forming an electric signal for
operating the actuator, the vehicle speed being controlled in
accordance with the condition of the electric signal;
timer means for detecting a time lapse after the commencement of
the cruise mode;
first control means for controlling the electric signal forming
means so as to obtain a fixed condition of the electric signal, to
quickly move the actuator means and prevent the actuator from being
moved back;
second control means for controlling the electric signal forming
means so that the condition of the electric signal is controlled by
feedback in accordance with a deviation of the actual vehicle speed
from the target vehicle speed; and
feedback gain control means for controlling feedback gain so that
the value thereof changes in accordance with a lapse of time from
the commencement of the feedback control.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an overall schematic view of the cruise control system
according to the present invention.
FIGS. 2, 2A, and 2B are flowcharts explaining the execution of the
routines in the control circuit in FIG. 1, in the first
embodiment.
FIGS. 3A and 3B show timing charts illustrating how the duty signal
is obtained.
FIG. 4 shows a relationship between the time lapse after
commencement of the cruise mode and the stroke of the throttle
valve actuator.
FIG. 5 shows a relationship between the time lapse and the values
of the feedback gain.
FIG. 6 shows a relationship between the time lapse and the vehicle
speed.
FIGS. 7A, 7B, 7B-1 and 7B-2 are flowcharts explaining the execution
of the routines in the control circuit in FIG. 1 in the second
embodiment.
FIG. 8 shows a relationship between the time lapse and the values
of feedback gain in the second embodiment.
FIGS. 9A and 9B show relationships between the time lapse and the
vehicle speed and between the time lapse and output torque.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, numeral 10 denotes an engine body, 12 an
intake manifold, 14 an intake pipe connected to the intake manifold
12, 16 a throttle valve, and 18 a transmission device for
connecting the crankshaft (not shown) of the engine body 10 to a
propeller shaft (not shown).
Reference numeral 20 denotes an actuator which responds to electric
signals for rotating the throttle valve 16 irrespective of the
depression of an accelerator pedal when the cruising apparatus is
in a cruising mode. The actuator 20 is provided with a diaphragm 22
which is connected, via a connecting member 24 such as a cable, to
a sector shaped lever 26 connected to a shaft 16a of the throttle
valve 16. A vacuum force applied to the diaphragm causes the
connecting member 24 to be wound around the outer surface of the
sector shaped lever, so that the throttle valve 16 is rotated. The
shaft 16a of the throttle valve 16 is connected to an accelerator
pedal (not shown) by a suitable and known connecting means, to
allow a desired control of degree of opening of the throttle valve
in accordance with a depression of the accelerator pedal when the
cruising mode operation is cancelled. When the apparatus is in the
cruising mode, the degree of opening of the throttle valve 16 is
controlled by the actuator 20, as will be described fully later.
The actuator 20 has a diaphragm chamber 28 on one side of the
diaphragm 22 remote from the connecting member 24. A spring 30 is
arranged in the diaphragm chamber 28 to urge the diaphragm to move
to the right in FIG. 1, so that the throttle valve 16 is closed.
The actuator 20 is provided with a relief valve 32 which
selectively opens or closes a relief port 34 opened to the
atmosphere. A spring 36 is provided to urge the relief valve 32 to
open the relief port 34 in normal operation, in such a manner that
the diaphragm chamber 28 is opened to the atmosphere. When a
solenoid mechanism 38 is energized, the valve member 32 is rotated
around an axis 32a against the force of the spring 36 so that the
relief port 34 is closed by the valve member 32. This allows the
vacuum pressure in the diaphragm chamber 28 to control a control
valve 40. The control valve 40 selectively opens or closes an
atmospheric air pressure port 42 or a vacuum pressure port 44. The
control valve 40 is urged by a spring 46 in such a manner that the
atmospheric air pressure port 42 is opened when the vacuum port 44
is closed. When a solenoid 48 is energized, the control valve 40 is
rotated around an axis 40a against the force of the spring 46 so
that the atmospheric air pressure port 42 is closed when the vacuum
port 44 is opened. This allows the diaphragm chamber 28 to be under
a vacuum pressure, causing the diaphragm 22 to be displaced to the
left in FIG. 1 against the force of the spring 30. Thus, the degree
of opening of the throttle valve 16 is controlled in accordance
with the level of vacuum pressure in the chamber 28. The vacuum
chamber 28 is connected, via vacuum passageways 50 and 50', to a
vacuum taking out port 54 in the intake pipe 14 of the engine. A
vacuum pump 52 is arranged between the vacuum passageways 50 and
50' to generate vacuum pressure for operating the vacuum actuator
20 when the vacuum pressure at the vacuum port 54 is weak. The
vacuum pump 52 is provided with a diaphragm 55, and a crank
mechanism 56 is connected to the diaphragm 55 for obtaining a
linear reciprocal movement of the diaphragm 55 from the rotational
movement of the crank mechanism 56. The rotation of the rotary
motor 58 causes the linear reciprocal movement of the diaphragm 55,
to generate a vacuum in the vacuum passageway 50. When the vacuum
pressure at the vacuum port 54 is high, the rotary motor 58 is
stopped, so that the vacuum passageway 50 is opened to the port 54
via a check valve 60. Thus, the passageway 50 is under a vacuum
pressure which is equal to that of the vacuum port 54.
A control circuit 64 is provided for controlling the operation of
the cruise control system according to the present invention, and
is constructed as a microcomputer system. The control circuit 64
comprises, as basic elements, a microprocessing unit (MPU), 66 of 8
or 16 bits, a memory 68, an input port 70, an output port 72, and a
bus 73 for connecting these elements. Various sensors and switches
are connected to the input port 70 for realizing the cruise control
according to the present invention. Among these sensors and
switches, those closely related to the present invention will now
be explained. A set switch 74 is manually operated by an operator
when starting the cruise mode operation, and this mode is commenced
when the switch 74 is moved from ON to OFF, i.e., when the switch
74 is once pushed and released. A cancel switch 76 is manually
operated by an operator for cancelling the cruise operation mode. A
vehicle speed sensor 78 is connected to an output shaft of the
transmission device 18 or to a rotational shaft of a vehicle speed
meter for generating pulse signals as the shaft rotates. A vehicle
speed SPN can be known from the distance between the pulse signals.
A vacuum switch 80 is provided to detect a predetermined value of
the vacuum level at the vacuum port 54. The selective operation of
the electric rotary motor 58 of the vacuum pump 52 is effected in
accordance with signals from the vacuum switch 80.
The output port 72 is connected to a transistor 82 for operating
the solenoid 38 and thus operating the relief valve 32, and to a
transistor 84 for operating the solenoid 48 and thus operating the
control valve 40. A down counter 86 is arranged between the output
port 72 and the transistor 84, to control a duration of a pulse
(duty ratio) in a pulse signal sent to the transistor 84 for
operating the control valve 40. The vacuum pressure level in the
diaphragm chamber 28 is controlled in accordance with the duty
ratio, whereby the degree of opening of the throttle valve 16 is
controlled.
The operation of the control circuit 64 will be described with
reference to flowcharts. In FIG. 2, when the routine is commenced,
an initialize routine is executed at step 90, where each of the
registers in the MPU 66, RAM area of the memory 66, input port 70,
and output port 72, and the like are initialized. At step 92 it is
determined if a predetermined waiting time of 48 milliseconds has
lapsed. In other words, the routine following step 92 is executed
at a time interval of 48 milliseconds. At step 94, it is determined
if a flag f.sub.SET is set. This flag is "0" when the vehicle is in
the normal running mode, and is "1" when the vehicle is in the
cruise mode. In the normal running mode, the routine goes from step
94 to step 96, where an actual value of the vehicle speed SPN which
is sensed by the vehicle speed sensor 78 is input. At step 98, it
is determined if the set switch 74 is ON. When the set switch 74 is
ON, the routine goes to step 100, where a set switch flag f.sub.SW
is set to "1". When the set switch 74 is OFF, the routine goes to
step 102, where it is determined if flag f.sub.SW is "1". A moment
of release of the set switch 74 which has been pushed, i.e., the
moment at which the set switch 74 is switched from ON to OFF,
corresponds to a timing for commencement of the cruise mode. In
this case the routine goes from step 102 to step 104, where the
flag f.sub.SET is set to "1" . At the next step 106, the value of
the actual vehicle speed SPN sensed by the sensor 78 is moved to a
RAM area of the memory 68, to store the target value of the
cruising speed SPM. At the following step 108, a high level signal
is sent to the transistor 82 to make it ON, thus energizing the
solenoid 38 and permitting the relief valve 32 to close the relief
port 34. As a result, the control of the degree of opening of the
throttle valve 16 by control of the vacuum level in the diaphragm
chamber 28 becomes possible. At the next step 110, a timer is
started. This timer detects predetermined times T.sub.1 and T.sub.2
after the commencement of the cruise operation, to control the
speed of movement of the diaphragm 22 so that the diaphragm quickly
reaches the position corresponding to the position of the throttle
valve 16 where the vehicle has a speed close to the target value
SPM.
In this embodiment of the present invention, T.sub.1 and T.sub.2
have values in a range between 0.5 to 1 second, and in a range
between 7 to 10 seconds, respectively, in accordance with the value
of the target vehicle speed SPM.
When the flag f.sub.SET =1, i.e., after the cruise mode has
commenced, the routine goes from step 94 to step 112, where an
actual vehicle speed SPN is input. Then, at step 114, a predicted
vehicle speed SS' is calculated. The predicted vehicle speed SS' is
a vehicle speed estimated at a time after a lapse of a
predetermined short period from the present time, which is
calculated from the present actual vehicle speed, by using the
following equation,
where Kv is a constant, and SPNX is an actual vehicle speed
obtained at the preceding cycle. By using this predicted vehicle
speed SS', a stable control can be attained irrespective of any
delay in the system in the control of the vehicle speed.
At step 116, a control operation of the vacuum pump 52 is realized.
This step includes operating the rotary motor 58 when it is
determined from the condition of the vacuum switch 80 that a vacuum
level at the vacuum port 54 is not sufficient to operate the
actuator 20. When the vacuum level at the port 54 sensed by the
vacuum switch 80 is high enough to operate the vacuum actuator, the
rotary pump 58 is stopped.
At step 118, it is determined if a time T.sub.2 has lapsed from the
commencement of the cruise mode. At the initial stage, if the
result of the determination at step 118 is "no", then the routine
goes from step 118 to step 119, where it is determined if a time
T.sub.1, which is selected from a range between 0.5 to 1 second,
has lapsed from the commencement of the cruise mode. When the time
T.sub.1 has not lapsed, then the routine goes from step 119 to step
120, where a predetermined maximum value of the duty ratio,
SDT.sub.max , is moved to SDT to store the duty ratio data in the
signal for operating the control valve 40. The selection of the
maximum value of the duty ratio allows the diaphragm 22 to quickly
move to a position corresponding to a position of the throttle
valve 16 at which the target value of the vehicle speed SPM is
obtained, as will be fully described later. At the next step 124,
the SDT value is sent and set to the down-counter 86, and the
down-counter 86 commences the count-down. As will be seen from
FIGS. 3a and 3b during the count-down, the down-counter 86 sends a
high level signal, i.e., "1" signal. After the count-down of the
SDT value is complete, the down-counter 86 sends a low level
signal, i.e., "0" signal. In other words, the duration of the "1"
signal from the down-counter 86 in relation to the time interval
for realizing the routine of FIG. 2 of 48 milliseconds corresponds
to the duty ratio SDT. As a result, the transistor 84 is made ON
for a time period which corresponds to the calculated duty ratio
SDT. Therefore, the control valve 40 opens the vacuum port 44 for a
period corresponding to the duty ratio SDT, to open the diaphragm
chamber 28 to the vacuum source 54 or 52 for a period corresponding
to the duty ratio SDT. As a result, the diaphragm chamber 28 is
subjected to a pressure which corresponds to the duty ratio SDT.
Thus, the diaphragm 22 takes a position which corresponds to the
calculated duty ratio SDT.
As described above, before the elapse of time T.sub.1 from the
commencement of the cruise mode, the value of the duty ratio SDT is
set to the maximum value SDT.sub.max, so that the speed of increase
in the vacuum pressure in the diaphragm chamber 28 becomes high
enough to allow the diaphragm 22 to move quickly to the left in
FIG. 1 to the position at which the throttle valve 16 has an
opening where the target SPM can be realized.
When the time T.sub.1 has elapsed at step 119 of FIG. 2, the
routine goes to step 126, where a feedback correction amount
.DELTA.SDT of the duty ratio is calculated by
wherein K is a first feedback gain having a value which is larger
than the value of a second feedback gain K', as will be explained
later (see FIG. 5). At step 128, the duty ratio SDT is calculated
as a sum of the value of SDT now stored, i.e., the duty ratio at
the preceding cycle, and the feedback correction amount
.DELTA.SDT.
When it is determined that the predetermined time T.sub.2 has
lapsed at step 118, the routine goes to step 130, where the
feedback correction amount .DELTA.SDT of the duty ratio is
calculated by
FIG. 4 schematically illustrates the relationship between the
stroke of the diaphragm 22 and the time lapse after the
commencement of the cruise mode, according to the above
construction of the present invention. Before the elapse of time
T.sub.1, the duty ratio SDT has the maximum value SDT.sub.max so
that the diaphragm 22 is quickly controlled to a stroke close to
but below the target stroke, as shown by a line l, to prevent an
unnecessary overshoot of the stroke. After the lapse of time
T.sub.1 but before lapse of time T.sub.2, a feedback control is
realized under the first gain K having a large value, so that the
diaphragm 22 is effectively controlled to the target stroke as
shown by a line m. After the lapse of time of T.sub.2, the
diaphragm 22 is controlled by the second gain K' so that it is
located close to the target value. In the prior art, the duty ratio
is controlled only to realize the maximum value during the period
of T.sub.1. Therefore, sometimes delay in the control of the target
value may occur, as shown by a line n, under a particular situation
of the system, where the position of the diaphragm at the lapse of
time T.sub.1 is far from the target position, causing the vehicle
speed to be temporarily decreased upon a transition from the normal
running mode to the cruise mode, as shown by a dotted line in FIG.
6. Conversely, according to the present invention, a drop in the
vehicle speed at the transition is effectively prevented, as shown
by the solid line in FIG. 6.
Note, the first feedback gain K after the predetermined time
T.sub.1 has lapsed from the commencement of the cruise mode can be
gradually decreased toward the second feedback gain K', as shown by
a dotted line p in FIG. 5.
In FIG. 2, at step 160 it is determined if a cancel condition is in
force. When the cruise mode is cancelled by, for example, making
the cancel switch 76 ON or by a depression of a brake pedal (not
shown), the routine goes from step 160 to step 162, where cancel
steps are executed, i.e., the relief valve 32 and the control valve
40 open the atmospheric air ports 34 and 42, respectively, which
causes the pressure the diaphragm chamber 28 to below the
atmospheric pressure, and thus moves the diaphragm 22 to the far
left in FIG. 1. At step 164, flags f.sub.SET and f.sub.SW are
reset.
A second embodiment described hereinafter differs from the first
embodiment in that a third feedback gain having a value between the
first feedback gain and the second feedback gain is provided in
order to obtain a constant torque of the engine when the system
enters a normal feedback control area of the second feedback gain
from the transient feedback control area of the first feedback
gain. FIG. 7 is a flow chart illustrating an execution of the
operation of the second embodiment of the present invention. This
flow chart is very similar to FIG. 2 in the first embodiment.
Therefore, only different portions thereof will be explained. Steps
92 to 110 executed when the system enters a cruise mode, to quickly
move the diaphragm 22 by selecting the maximum duty ratio value,
steps 119, 120, and 124 executed first during the feedback mode for
taking the first value of feedback gain K, and steps 126 and 128
executed during the feedback mode under the lower second feedback
gain K' are the same as the corresponding steps in FIG. 2 in the
first embodiment.
As in the first embodiment, before the elapse of the time of
T.sub.2 from the commencement of the cruise mode, the diaphragm 22
is moved to a position near the target stroke by setting the duty
ratio SDT to the maximum value SDT.sub.max, by executing steps 119
to 124. After the time T.sub.2 has elapsed from the commencement of
the cruise mode at point 118, the routine goes to step 200 where it
is determined if a feedback gain control flag f.sub.t is set. This
flag f.sub.t is initially cleared (0), so that a "No" at step 200
means that the time T.sub.1 has just elapsed from the commencement
of the cruise mode. In this case, the routine goes to step 202,
where this flag f.sub.t is set (1), and to step 204 where a
feedback gain correction amount .alpha. is calculated by
where n is a weight factor. Then, at the following step 206, the
duty ratio correction amount .DELTA.SDT is calculated by the
following equation.
In this case, a feedback gain K' as calculated has, as shown in
FIG. 8, a third value between the first gain K and the second gain
K'. The duty ratio correction value .DELTA.SDT as calculated at
step 206 is, in view of the .alpha.calculated at step 204,
expressed by ##EQU1## wherein k"=(1/n) .times.(K-K')+K', and the
value of n is selected so that the value of the third gain k" is
between the values of the first gain K and second gain K'.
At the following cycle, the result at step 200 is "Yes", since the
flag f.sub.t is set at the preceding cycle at step 202. Thus, the
routine goes to step 135, where the feedback gain correction amount
.alpha. becomes zero. Therefore, the feedback gain at step 206 has
the second value K'. As will be clear from the above, the feedback
gain temporarily has an intermediate third value K" when the
above-mentioned period T.sub.2 has just elapsed, and then has the
second value K', as will be seen from FIG. 8.
FIGS. 9a and 9b show the change of actual vehicle speed and engine
torque, respectively, in the second embodiment, after the cruise
mode operation has commenced, in comparison with the first
embodiment. In the first embodiment, the feedback gain is directly
changed from the first value K to the second value K' upon the
lapse of time T.sub.2, as already explained in relation to FIG. 5.
Due to the large drop in value of the feedback gain, the engine
torque is temporarily decreased, as shown by a dotted line, causing
a drop in the vehicle speed, as shown by the dotted line. Contrary
to this, according to the second embodiment, the feedback gain
temporarily has the third value K" intermediate between
the first and second values K and K', when the time T.sub.2 has
just elapsed. Therefore, an abrupt change in feedback gain is
prevented, so that a drop in the engine torque and vehicle speed
does not occur.
Although the present invention is described with reference to the
attached drawings many modification and changes can be made by
those skilled in this art without departing from the scope and
spirit of the present invention.
* * * * *